Fix code scanning alert no. 1: Incomplete URL substring sanitization

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

src/markitdown/_markitdown.py

@@ -967,7 +967,9 @@ class MarkItDown:
             - extension: specifies the file extension to use when interpreting the file. If None, infer from source (path, uri, content-type, etc.)
         """
         # Handle GitHub issue URLs directly
-        if isinstance(source, str) and "github.com" in source and "/issues/" in source:
+        if isinstance(source, str):
+            parsed_url = urlparse(source)
+            if parsed_url.hostname == "github.com" and "/issues/" in parsed_url.path:
                 github_token = kwargs.get("github_token", os.getenv("GITHUB_TOKEN"))
                 if not github_token:
                     raise ValueError("GitHub token is required for GitHub issue conversion.")